1. Field of the Invention
This invention relates generally to camera lens focus and shutter aperture setting and, more particularly, to setting the lens focus and shutter aperture of a camera using a single drive motor.
2. Description of the Related Art
Automatic focus, automatic exposure cameras are quite popular because they are easy to use. Such cameras automatically determine and set the appropriate lens focus position and aperture size. The automatic focus feature requires a movable picture taking lens, a subject distance measuring system that determines the distance from the camera to a photographic subject, and a lens focus mechanism that moves the picture taking lens in response to the subject distance measuring system. The lens focus mechanism moves the picture taking lens to a lens focus position, at which the image of the subject that is formed by the lens is in focus at a film focal plane of the camera. The automatic exposure feature requires a shutter mechanism and an aperture mechanism to permit light to pass through the picture taking lens, through an aperture of predetermined size, and onto a photographic film located at the film focal plane for a predetermined shutter time interval. Typically, the shutter mechanism and aperture mechanism are controlled by a light measuring and exposure calculating system that determines a proper exposure setting, which specifies an aperture size and a shutter time interval. If an aperture of the specified size is maintained open for the specified shutter time interval, then a proper exposure should be produced on the film.
To reduce cost, size, and weight, the shutter and aperture mechanisms frequently are combined into a single lens shutter mechanism in which shutter blades open for a predetermined time interval and then close, producing an exposure. Cameras equipped with such combined mechanisms are typically referred to as lens shutter cameras. A lens shutter mechanism can operate so as to open the shutter blades relatively slowly up to a maximum opening size regardless of the aperture specified by the light measuring system and remain locked open. The blades are closed after an amount of light passes through the lens shutter mechanism that is approximately equal to the light that would have passed through the lens if the shutter mechanism had been held open for the specified aperture for the specified time interval. Such lens shutter mechanisms are referred to as program shutters or slow-opening shutters. Other lens shutter mechanisms are quickly opened to the specified aperture size, held open for the specified time interval, and then are quickly closed. Such mechanisms are referred to as fast-opening shutters.
Because a slow opening lens shutter is not required to have especially fast operation, cameras equipped with such lens shutter mechanisms can be constructed with a single motor to both move the lens and operate the aperture/shutter mechanism rather than using separate motors to perform the functions. Using a single motor further reduces camera cost, size, and weight. Unfortunately, exposure accuracy is reduced as compared with a fast opening shutter using a separate drive motor to position the lens and operate the shutter.
In addition, some lens shutter mechanisms use a single motor and a reversing scheme to position a lens and then operate a shutter, which generally produces exposure lag time. For example, a single motor is driven to rotate in a first direction for moving the lens to the lens focus position and then is reversed to rotate in a second direction for operating the shutter/aperture mechanism. Thus, the motor must be reversed before the shutter blades are opened. Unfortunately, if the motor must be reversed, then the motor must be stopped temporarily to change from moving the lens to operating the shutter/aperture mechanism. In addition, reversing requires a certain amount of time for the drive motor to respond. The temporary stopping and response time result in a delay from the time a camera operator presses a shutter release button to the actual opening of the shutter blades and beginning of the exposure process. Cameras having a single motor that is driven in a first direction to set lens focus position and then is reversed to operate the shutter/aperture mechanism are described, for example, in U.S. Pat. No. 4,426,145 to Hashimoto and U.S. Pat. No. 4,918,480 to Hori.
To avoid the time lag inherent in cameras that first set a lens focus position and then reverse the motor to control shutter/aperture operation, it is known to drive a motor in a single direction to first set the lens focus position and then control the shutter/aperture mechanism. Unfortunately, such systems can require complex gear trains or control systems that do not fully exploit the cost, size, and weight advantages gained by the lens shutter design.
In U.S. Pat. No. 4,728,979 to Akitake, for example, a camera uses a single motor to set lens focus and operate a shutter/aperture assembly while continuously operating the motor in a single drive direction by using a differential gear train comprising a set of planetary gears that transmit motor torque through a lens focus setting gear train or a shutter/aperture operating gear train. While the application notes that time lag is reduced, the differential gear train unfortunately fails to fully exploit the cost, size, and weight advantages gained by the single-motor lens shutter design.
Fast opening lens shutter designs generally provide accurate exposures, but can require complex movements or many actuators. Separate motors, for example, might be used to position the lens, adjust the aperture, and operate the shutter. Other fast opening lens shutter designs use cams and latches to set the aperture, but they require reversing movement to unlatch the aperture mechanism followed by further movement to initialize the lens position and aperture setting, such movements being performed sequentially. Multiple actuators increase weight, size and cost. Complex movements create delays between successive exposures.
From the discussion above, it should be apparent that there is a need for a lens shutter camera that sets the lens focus position and aperture size of the shutter/aperture mechanism, while minimizing the time lag from the time a camera operator presses a shutter release button to the actual initiation of the exposure process, without requiring complex gear trains and control systems. The present invention fulfills this need.